You should always try to feed your baby when you think it's hungry or at certain times. So yes you should try and feed your baby every 3 hours.
If your baby is hungry he or she will take to the bottle or nursing but if they're not hungry then they will not take to the bottle or nursing and you'll know if they are hungry or not.
Also a baby will let you know when they're full by pushing the bottle away from his/her lips or remove their self from nursing.
Your baby should also begin to cry when they're hungry and this will happen in the middle of the night as well.